5.1.8 Dynamic Printf

The dynamic printf command dprintf combines a breakpoint with
formatted printing of your program’s data to give you the effect of
inserting printf calls into your program on-the-fly, without
having to recompile it.

In its most basic form, the output goes to the GDB console. However,
you can set the variable dprintf-style for alternate handling.
For instance, you can ask to format the output by calling your
program’s printf function. This has the advantage that the
characters go to the program’s output device, so they can recorded in
redirects to files and so forth.

If you are doing remote debugging with a stub or agent, you can also
ask to have the printf handled by the remote agent. In addition to
ensuring that the output goes to the remote program’s device along
with any other output the program might produce, you can also ask that
the dprintf remain active even after disconnecting from the remote
target. Using the stub/agent is also more efficient, as it can do
everything without needing to communicate with GDB.

dprintf location,template,expression[,expression…]

Whenever execution reaches location, print the values of one or
more expressions under the control of the string template.
To print several values, separate them with commas.

set dprintf-style style

Set the dprintf output to be handled in one of several different
styles enumerated below. A change of style affects all existing
dynamic printfs immediately. (If you need individual control over the
print commands, simply define normal breakpoints with
explicitly-supplied command lists.)

gdb

Handle the output using the GDBprintf command.

call

Handle the output by calling a function in your program (normally
printf).

agent

Have the remote debugging agent (such as gdbserver) handle
the output itself. This style is only available for agents that
support running commands on the target.

set dprintf-function function

Set the function to call if the dprintf style is call. By
default its value is printf. You may set it to any expression.
that GDB can evaluate to a function, as per the call
command.

set dprintf-channel channel

Set a “channel” for dprintf. If set to a non-empty value,
GDB will evaluate it as an expression and pass the result as
a first argument to the dprintf-function, in the manner of
fprintf and similar functions. Otherwise, the dprintf format
string will be the first argument, in the manner of printf.

As an example, if you wanted dprintf output to go to a logfile
that is a standard I/O stream assigned to the variable mylog,
you could do the following:

Note that the info break displays the dynamic printf commands
as normal breakpoint commands; you can thus easily see the effect of
the variable settings.

set disconnected-dprintf on

set disconnected-dprintf off

Choose whether dprintf commands should continue to run if
GDB has disconnected from the target. This only applies
if the dprintf-style is agent.

show disconnected-dprintf off

Show the current choice for disconnected dprintf.

GDB does not check the validity of function and channel,
relying on you to supply values that are meaningful for the contexts
in which they are being used. For instance, the function and channel
may be the values of local variables, but if that is the case, then
all enabled dynamic prints must be at locations within the scope of
those locals. If evaluation fails, GDB will report an error.